The present invention relates in general to an apparatus and method for suppressing electromagnetic emissions from computer equipment. More particularly, the present invention relates to an apparatus and method for suppressing electromagnetic emissions generated by the operation of electrical discharge type printers.
Electrical discharge printers are well known in the prior art. Typically, such printers provide many performance characteristics of complex and costly high speed printers at a price below low speed models. Such printers allow printing speeds above one thousand characters per second on electrosensitive paper. Typical examples of such electrical discharge printers are the models 1110 and 1180 produced by SCI Systems, Inc.
Electrical discharge printers find application in many modern computer systems requiring a low cost, high speed output device. A typical application of such a printer would be its use in conjunction with a document processor such as a Burroughs S3000 system, which reads printed information on checks and sorts the checks into a plurality of pockets based on the information read. When a pocket has been filled with checks, it is desirable to have a printed record of pertinent information on the checks in the filled pocket. Thus, after the pocket has been filled with checks, the printer produces a printed record describing the data on the checks in the pocket.
Electrical discharge printers utilize a spark mechanism in order to accomplish printing on electrosensitive paper. The sparks produced are characterized as having a very sharp rise time and result in a multitude of discharges being generated. These discharges generate a broad band of frequency which is generated through all of the surrounding medium right in the area of the printer and thus induces energy in the surrounding medium. In a typical electrical discharge printer, the nearest medium to the printer is the previously printed electrosensitive paper itself. The energy generated by the electrical discharges is thus coupled into the metallized coating of the electrosensitive paper and once on the coating, the paper acts as an antenna because of its conductive coating. The energy radiates along the surface of the paper, out of the machine and right into the open air.
The Federal Communications Commissions rules impose limitations on the levels of radio frequency energy that may be generated by computer equipment. The purpose of these rules is to assure that computer equipment does not generate energy which can interfere with radio communications.
As used in prior art systems, the electrosensitive paper fed out from the printer provides a continuous (but not perfect) conductor. Thus, the paper acts as a relatively good antenna.
In the prior art, several techniques have been utilized or proposed for suppressing the antenna effect created by the ejected length of electrosensitive paper which is produced as the printing operation occurs.
One prior art technique involves placing the printer and the printed paper output in a shielded EMI enclosure. This technique does not eliminate the radiated energy, but contains it within the shielded container. This technique of containing the electromagnetic radiation is quite effective. However, the cost of providing such an enclosure and the inconvenience to the operator make this technique undesirable. Thus, after a length of paper has been printed and is to be removed from the printer, the operator must open a drawer or other mechanism to remove the printout and thereafter close the drawer and latch any interlock provided prior to starting operation again.
Another prior art technique of reducing the antenna effect of the printed paper is to cut the length of paper output at predetermined intervals. This effectively shortens the length of the antenna and thereby reduces the amount of electromagnetic radiation generated. This technique suffers from the fact that the paper may have to be cut at an interval which is shorter than the length required to contain a single printout for a full pocket. For example, in the Burroughs S3000 document processor, a full pocket of documents contain approximately 300 checks. A single printout of data corresponding to the 300 checks would be too long to produce without the paper producing undesirable antenna effects. Thus, the length of paper corresponding to the printout for the 300 checks would have to be cut at one or more places. This is undesirable.
Other techniques used to suppress EMI in electrical discharge printers involve various combinations of grounding the electrosensitive paper with wipers and placing the printer in a metallized enclosure. Providing grounding wipers affects the reliability of the printer's paper feed mechanism and results in the requirement of considerable maintenance in order to maintain the EMI suppression. Therefore, such techniques are both unreliable and expensive.
It is the general object of the present invention to overcome these and other drawbacks of the prior art by providing a method and apparatus for electromagnetic interference suppression in electrical discharge printers.
It is a further object of the present invention to provide an appartus and method which effectively reduces the radiation level produced by the printed electrosensitive paper from an electrical discharge printer.
It is another object of the present invention to provide a low cost method of modifying electical discharge printers in order to reduce electromagnetic energy generated by the operation of such printers.
It is still another object of the present invention to provide a method of operating an electrical discharge printer whereby electromagnetic interference generated by the printer is suppressed without the need of enclosing the printer or printed output in a shielded container.
It is an additional object of the present invention to reduce the electromagnetic energy radiated via the printed electrosensitive paper output by an electrical discharge printer without the need to cut the paper at predetermined intervals.
It is still a further object of the present invention to provide a method of reducing radio frequency energy generated by an electrical discharge printer to levels below the maximums permitted by regulatory agencies such as the Federal Communications Commission (FCC).
These and other objects, features and advantages of the present invention will become more apparent from the detailed description of the preferred embodiment when read in connection with the drawings.